Abstract: A functional connection, evidenced by synergism and cross-tolerance, seems to exist between the μ-opioid and the α2-adrenergic receptors in the superficial dorsal horn of the rat spinal cord. In this study possible cross-antagonism between these systems was studied by intrathecal administration of morphine 2.5 μg or dexmedetomidine 1.25 μg 15 min. after subcutaneous injection of naloxone 1.0 mg/kg or atipamezole 3.0 mg/kg in the morphine group and 4.0 mg/kg or 1.0 mg/kg in the dexmedetomidine group, respectively. No cross-antagonism of the antinociceptive effect was seen either in the tail flick or in the hot plate test at 15, 30, 45 and 60 min. from the pretreatment.